Observation of the ${\mathrm{\Lambda }}_{\mathrm{b}}^0$ $\to$ J/$\psi \mathrm{\Lambda }\varphi$ decay in proton-proton collisions at $\sqrt{s}=$ 13 TeV

The observation of the ${\mathrm{\Lambda }}_{\mathrm{b}}^0$ $\to$ J/$\psi \mathrm{\Lambda }\varphi$ decay is reported using proton-proton collision data collected at $\sqrt{s}=$ 13 TeV by the CMS experiment at the LHC in 2018, corresponding to an integrated luminosity of 60 fb${}^{-1}$. The ratio of the branching fractions $\mathcal{B}({\mathrm{\Lambda }}_{\mathrm{b}}^0$ $\to$ J/$\psi \mathrm{\Lambda }\varphi )$ $\mathrm{/}$ $\mathcal{B}({\mathrm{\Lambda }}_{\mathrm{b}}^0$ $\to$ $\psi \mathrm{\Lambda })$ is measured to be (8.26 $\pm$ 0.90 (stat) $\pm$ 0.68 (syst) $\pm$ 0.11 $(\mathcal{B}))$ $\times$ 10${}^{-2}$, where the first uncertainty is statistical, the second is systematic, and the last uncertainty reflects the uncertainties in the world-average branching fractions of $\varphi$ and $\psi$(2S) decays to the reconstructed final states.